The present invention relates to an improved process for the preparation of high quality amine oxides from secondary and tertiary aliphatic amines. More particularly, the present invention relates to an improved process for the preparation of amine oxides from secondary and tertiary aliphatic amines useful in the preparation of hair conditioners and shampoos, toothpaste, laundry detergent powder, fabric softeners, toilet soap bars and cosmetics, surfactants as well as in other applications as synthetic intermediates and excellent spin trapping reagents.
The N-oxides holds a key position in the chemistry of heterocycles as well as in biomedical area. The tertiary amine oxides are widely used in treatment of fabrics and preparation of hair conditioners and shampoos, toothpaste, laundry detergent powder, fabric softeners, toilet soap bars and cosmetics as well as in other applications. They were also used as stoichiometric oxidants in metal catalysed hydroxylation and epoxidation reactions of olefins. On the other hand, the oxides derived from secondary amines, called nitrones are highly valuable synthetic intermediates and excellent spin trapping reagents. In particular nitrones are excellent 1,3 dipoles and have been utilized for the synthesis of various nitrogen containing biologically active compounds e.g. alkaloids and lactams.
Conventionally tertiary amine oxides are prepared by oxidation of respective tertiary amines with strong oxidising agent like aqueous hydrogen peroxide in a solvent such as water, lower alcohol, acetone or acetic acid. A dilute or preferably concentrated (30-90% by weight) hydrogen peroxide solution is added in stoichiometric or greater amount to an aqueous solution containing the tertiary amine to obtain amine oxide, (U.S. Pat. NO. 3,215,741). The drawback is that the reaction transforms into a gel resembling a thick paste long before completion of the reaction, which retards further reaction. The yields are only 30-40% by weight of amine oxide. Later several methods such as incorporation of catalyst and/chelating agent have been developed to in order to increase the quality and yields of the product.
In case of secondary amines, the classical methods involve the condensation of N-monosubstituted hydroxylamines with carbonyl compounds or the direct oxidation of N,N-disubstituted hydroxylamines. Later direct oxidation of secondary amines using several oxidising systems such as R2C(xcexc-O2), Na2WO4xe2x80x94H2O2, SeO2, TPAP-NMO land UHP-M (M=Mo, W), MTOxe2x80x94H2O2 have been developed to accomplish nitrones under homogeneous conditions. The drawback in all the above cases is the difficulty in recovering the homogeneous catalyst/reagents from the reaction mixture.
Reference may be made to a U.S. Pat. No. 3,283,007 wherein the oxidation of tertiary amines using diethelene triamine penta/tetra acetic acid as chelating agent and sometimes contaminated with heavy metals is recommended to improve the yield. The hydrogen peroxide solution employed has concentration of at least 30-75% by weight. The disadvantages of this process are high reaction temperatures ranging between 40-100xc2x0 C., longer reaction periods, and lower yields of amine oxides.
Reference may be made to U.S. Pat. No. 3,424,780, wherein high yields of tertiary amine oxides are achieved by carrying the oxidation of tertiary amine with 30-70% by weight of aqueous hydrogen peroxide using 0.01 to 2% weight of carbondioxide, in presence of a chelating agent, tetra acetylene diamine, a salt thereof, polyphosphates, stannates, a hydroxy carboxylic acid salts or the salt of poly carboxylic acid. The reaction is carried out at a temperature ranging from 40 to 80xc2x0 C. The disadvantages of this process are high reaction temperature, longer reaction periods and the amine oxide formed is intensively coloured when carbon dioxide atmosphere is used to speed up the reaction and this method necessitates injecting a gas which requires handling facilities. Another disadvantage is more than 30% by weight of hydrogen peroxide is not environmentally friendly.
Reference may be made to another U.S. Pat. No. 4,889,954 wherein the tertiary amines are reacted in high yields to give the corresponding amine oxides with a low content of nitrosamine, the oxidation of tertiary amine being carried out in the presence of a dialkyl carboxylic acid ester as catalyst and if appropriate, ascorbic acid as a co-catalyst using 45-70% by weight of hydrogen peroxide. The drawbacks in the above process are the requirement of frequent addition of water to avoid gel formation, high reaction temperatures, longer reaction periods and difficulty in separation of the catalyst from the reaction mixture.
Reference may be made to another U.S. Pat. No. 4,565,891 wherein octacyano molybdate or iron salts are used as catalysts and molecular oxygen for oxidation of tertiary amines at high pressures and temperatures. The main drawback of this process is the need of very high temperature of 90-130xc2x0 C. and very low yields of amine oxide reporting 11-52% of conversion.
Reference may be made to a U.S. Pat. No. 5,130,488 wherein the solid amine oxide can be prepared by reacting a tertiary amine with hydrogen peroxide using carbon dioxide in presence of acetate and cooling to precipitate the product. This process is superior to previously known methods of preparing amine oxides. However, its use can sometimes lead to cleavage of the solvents, plating on the walls of the vessel used for the precipitation, contamination of the product with residual peroxide, and or discoloration of the product.
Reference may be made to a publication by Walter W. Zajac et al., J. Org. Chem.; 53, 5856, 1988 wherein the oxidation of secondary and tertiary amines using 2-sulfonyloxyxaziridines (Davis Reagents) were reported. The drawback of the above process is, the reagent was used in stoichiometric amounts.
Reference may be made to a publication by Shun-Ichi Murahashi et al., J. Org. Chem.; 55, 1736, 1990 wherein the sodium tungstate was used as catalyst for the oxidation of secondary amines. The drawback is the difficulty in recovery of the catalyst from homogeneous conditions.
Another reference may be made to publication by Murraay et al., J. Org. Chem.; 61, 8099, 1996 wherein methyltrioxorhenium was used as a catalyst in oxidation of secondary amines. The drawback is the difficulty in recovery of the catalyst.
The main object of the present invention is to provide an eco-friendly and simple process for N-oxidation of secondary and tertiary amines using layered double hydroxides exchanged with anions of transition metal oxides as a catalyst, which is cheaper, non-corrosive and recyclable catalyst utilising only lower percentage of hydrogen peroxide at room temperatures to give high yields of product.
Another object of the present invention is to provide an improved process for the preparation of tertiary amine oxides and secondary amine oxides (nitrones), widely used in detergents, shampoos, fabric softers and biomedical area.
Another object of the present invention is the use of non-corrosive and low cost heterogeneous catalyst i.e. layered double hydroxides exchanged with tungstate, molybdate, vanadate and their polyanions.
A further object of the invention is to provide a environmentally friendly process for the preparation of tertiary amine oxides and nitrones, using water alone or in combination with dodecylbenzenesulfonic acid sodium salt as an additive.
Accordingly, the present invention provides an improved process for the preparation of amine oxides of a very high quality which comprises reacting tertiary and secondary amines with hydrogen peroxide as an oxidant in presence of a recyclable heterogeneous catalyst, layered double hydroxides exchanged with anion of transition metal oxides, with tungstate, molybdate, vanadate, and their polyanions i.e. polyoxometallates, in a solvent selected from water, water containing dodecylbenzzenesulfonic salt additive or a water miscible organic solvent at a temperature ranges between 10-25xc2x0 C. for a period of 1-6 hours under continuous stirring and separating the product by simple filtration and subsequently evaporation of solvents by known methods.
IN an embodiment of the present invention, the heterogeneous catalyst used is the layered double hydroxides exchanged with transition metal oxides selected from a group consisting of tungstate, molybdate, vanadate and their polyanions i.e. polyoxometalates. Having formula I: [MII(1-x)MIIIx(OH)2][Mnxe2x88x92]x/2.zH2O, which is derived from LDH having formula II [MII(1-x)MIIIx(OH)2][Anxe2x88x92]x/2.zH2O where M is a transition metal oxides selected from the group consisting of W, Mo, V and Anxe2x88x92 is interstitial anion, selected from nitrate, chloride and MII is a divalent cation selection from the group consisting of Mg2+, Mn2+, Fe2+, V2+, Co2+, Ni2+, Cu2+, Zn2+ Pd2+, or Ca2+ and MIII is a trivalent ion selected from the group consisting of Al3+, Cr3+, V3+, Mn3+, Fe3+, Co3+, Ni3+, Rh3+, Ru3+, Ga3+ or La3+.
In another embodiment of the present invention, the tertiary amines used are having the general formula R1R2NR3 wherein R1, R2 and R3, which may be the same or different, and are the straight-chain or branched-chain groups selected from alkyl, alkenyl and aralkyls having C1-C24 carbons selected from N,N-dimethyl decyl amine, N,N-dimethyl dodecyl amine, N,N-dimethylbenzylamine, triethylamine, tributylamine and cyclic amines selected from imidazolines pyridines, N-substituted piperazines, N-substituted piperadines or N-substituted morpholines, e.g., N-methylmorpholine.
In another embodiment of the present invention, the secondary amines used are having general formula R1R2NH wherein R1 and R2 may be the same or different and are the straight-chain or branched-chain groups selected from alkyl, alkenyl and aralkyls having C1-C24 carbons, selected from dibutyl amine, dibenzyl amine, N-benzyl phenethylamine, N-phenyl benzylamine and cyclic amines selected from piperidine, 1,2,3,4, tetrahydro isoquinoline.
In another embodiment of the present invention aqueous hydrogen peroxide is added slowly in a controlled manner for a period ranges between 0-120 min.
In yet another embodiment of the present invention, the catalyst introduced in the system is 6-12% by weight of anion of transition metal oxides selected from tungstate, molybdate, vanadate and their polyanions as polyoxometalates.
In still another embodiment of the present invention, water miscible organic solvents selected from group consisting of methonol, ethanol, isopropanol, 1-propanol, 1-butanol, 2-butanol and isobutyl alcohol are used.
In still another embodiment of the present invention, the amount of hydrogen peroxide used is 2 to 6 moles per mole of amine.